Dynacord SL1200 LPN-Filter

[Koobenot]

Hello, I'd like to ask what's theoretically doing the LPN-Filter switch on this power amplifier? Here's the description in the manual:

The patented LPN (Low-pass notch) filter corrects the frequency and phase response of the connected loudspeakers. This effect can not be reached using equalizers or „Bass-Boosters”, because the LPN filter mainly optimizes the rise time of the audio signal. Switch the filter ON or OFF for evaluation of the actual effect that the filter has on the sound. The sound is getting more powerful, especially when using full-range speakers (e.g. 12/2, 15/2) or subwoofers. For large systems (e.g. controlled via DC‘s DSP 260) using theLPN filter is not recommended.

As far as I understand that's how it looks like:

So let's imagine we're filtering out this exact frequency at 50Hz with the given Q-factor. But why? The amplifier doesn't "know" or cannot detect the resonant frequency of the passive loudspeaker, nor it can change the frequency dynamically, since it's a physical switch on the amplifier itself. Which frequency range are we exactly notching and how is this correlated to phase in the context of power amplifiers and loudspeakers? Is it that for instance we have a low-pass filer at 20kHz with let's say 96dB/octave cutoff slope and then the notch itself is upwards so that it can compensate for the loss of highs at the cutoff frequency?

 

[staticV3]

If it really is a fixed 50Hz Notch filter, then that's pretty useless.

 

[Koobenot]

The manual doesn't say cut-off frequency, nor cut-off slope.

 

[Koobenot]

Given that the amplifier outputs are only two, they will send the entire frequency band to the loudspeakers (20Hz-20kHz, relatively speaking). What will this notch filter do then? Let's say it will eliminate my 50Hz hum on the power supply, but that's a design issue and not something I should have to worry about as a sound guy. Or will it filter out some resonant frequency, but how will it know what it is, since it depends on the sounding body and is a dynamic quantity? The other thing that came to my mind is that this notch should be very low, say 20-30Hz with a super narrow Q and to eliminate any infra-low frequencies that only "waste electricity" and cannot actually be reproduced by the speakers. I have seen the same trick with the mastering limiters in the software, where there is a switch with a very steep cutoff (96/128 dB/oct or brickwall) and eliminates everything below 20Hz, let's say. I also payed attention to the following sentence from the quoted text:

... the LPN filter mainly optimizes the rise time of the audio signal.

Let's say there is some delay between left/right resulting in "dephasing" of the speaker movement in the loudspeakers, this switch will correct the phase, but how does this technically happen? Based on the current drawn by the speaker there is a comparator circuit that compares left/right and causes one side to "lag" relative to the other so that they are calibrated and playing in phase? Sounds like science fiction to me for a simple mid-range amplifier ...

Of course, I've also emailed Dynacord, so we'll see what it's all about. Of course, A/B testing the button might yield some empirical result on the "with/without I like it better" principle, but I want to know what's actually going on behind that button.

 

[Koobenot]

Got in touch with one of Dynacord's engineers. So here's what happens:

When the button is pressed, the input signal is split into two - one copy reproduces the flat signal, while the second copy activates the band-pass/notch filter in parallel, in the 20Hz-150Hz range, and applies a linear-phase correction so that there is no variation in the lows relative to the main signal and, furthermore, the EQ-to reduce the wavelength. This results in "boosting" the attack of the wavelength and can easily be heard with some "bass-heavy" music - electronic, rap, hip-hop, etc. Finally, the two signals are summed back together and driven to the left and right side outputs. The reason it's a band-pass is to filter out anything below 20Hz, which is unnecessary, disgusting, just wastes current and makes the speaker wheeze, and therefore anything above 150Hz - the two component filters, one low-cut, the second high cut. The result is a "snappier" and louder bass, as they also mention in the manual, and you may have guessed, this is not equivalent to a simple bass boost in the same area, as they also have already written, just perfect for small scale systems - 2x12.2, 2x15.2, 2x18.2, etc.

I decided to simulate the filter behavior with a theoretical setup in Reaper with FabFilter Pro-Q 2 and Voxengo Span, when running pink noise and band-pass filter in parallel, linear-phase mode yields interesting results.